I sorta agree with that last post, what I've started to do in previous rps is just make bland characters and then let the story dictate what I do or how amazing my character becomes.  So far it hasn't let me down, except in combat-style rps which I rarely do anymore, because I try everything I write out on my kids first (I'm running out of them) and I realized most people who write in these type of forums are better and tactic thinking then my 14 year old.  (He just sorta squeals and frowns at me) 

Anyway, I digress.  Mary Sue characters are often abandoned because there just isn't as much progression as people want- they get boring just doing the same thing over and over.  It's like buying the last move in a progressive game first- then just playing the game using the same move over and over again. Sure, you'll beat most everything you come up against the repetition is just painstaking.